It is known that modern internal combustion engines normally include a fuel injection system provided for injecting metered quantities of fuel into the engine, namely into the engine combustion chambers. Particularly for Diesel engines, the fuel injection system usually includes a low-pressure fuel pump, which receives fuel from a fuel tank and delivers the fuel into a low-pressure line, and a high-pressure fuel pump, which draws the fuel from the low-pressure fuel line and delivers the fuel into a high-pressure fuel rail. The high-pressure fuel rail is in fluid communication with one or more fuel injectors, each of which is arranged to inject the fuel directly into a corresponding combustion chamber of the engine.
Both the low-pressure fuel pump and the high-pressure fuel pump are usually driven by the engine crankshaft via transmission chains or belts, so that they actually continue to pump fuel as long as the engine is operating. For this reason, the fuel injection system is also provided with a number of valves, which are used to regulate the fuel circulation in response to the different engine operating conditions. These valves may include a first controllable valve, which is disposed at the inlet of the high-pressure fuel pump and a second controllable valve, which is disposed in a return line that fluidly connects the fuel rail to the fuel tank.
The first controllable valve may be an on/off valve, which is opened during the induction stroke of the high-pressure fuel pump and closed during the subsequent compression stroke. In this way, by regulating the timing between the closing of the valve and the end of the compression stroke, the fuel quantity delivered by the high-pressure fuel pump is efficiently adjusted. In order to compensate for the different quantities of fuel delivered by the high-pressure fuel pump, an additional overflow valve is normally disposed in the low-pressure fuel line to discharge part of the fuel coming from the low-pressure fuel pump back into the fuel tank.
The first and the second controllable valves may be electromechanically actuated valves controlled by an electronic control unit (ECU), which is generally configured to determine, on the basis of the engine operating conditions, a target value of the fuel rail internal pressure and to operate these valves in order to follow up said pressure target value.
Under fuel cut-off conditions, namely when the fuel injectors are closed and no fuel is injected into the engine combustion chambers, the ECU is normally configured to operate the first valve so that no additional fuel is supplied by the high-pressure fuel pump into the fuel rail, and to adjust the position of the second valve with a closed loop control logic aimed to progressively reduce the fuel rail internal pressure down to a minimum value thereof.
More specifically, the first valve, which is disposed at the inlet of the high-pressure fuel pump, is kept open both during the induction stroke and during the compression stroke of the pump, so that all the fuel drawn from the low-pressure fuel line is sent back to the pump inlet. To deal with this counter-flow of fuel, the overflow valve is conventionally integrated in the high-pressure fuel pump, so that the fuel coming back from the high-pressure fuel pump is immediately discharged into the fuel tank. This solution is quite effective, but the “integrated” high-pressure pump is becoming too heavy and expensive for modern engines, which are designed to reduce weights and costs as much as possible.
For this reason, some pump suppliers are proposing to realize the high-pressure fuel pump and the overflow valve as two separated components and to connect them by means of an intermediate line, thereby allowing the high-pressure fuel pump to be optimized both in term of weight and cost. However, when the first controllable valve is kept open under a fuel cut-off condition, this layout is unable to immediately discharge the fuel that comes back from the high-pressure fuel pump, thereby causing significant pressure fluctuations in the intermediate line. These pressure fluctuations, which are particularly intense for high values of the engine speed, may generate noises and mechanical stresses.